Published patent application WO2010/116294 discloses embodiments of phosphor-enhanced light source wherein a stack with a layer with an inorganic luminescent material and with a layer with an organic luminescent material are arranged in a so-termed direct phosphor configuration or in a so-termed remote phosphor configuration. In a direct phosphor configuration the stack of layers with the different luminescent materials is applied directly on top of a light emitting surface of a light source. In a remote phosphor configuration there is a gap between the light emitting surface of the light source and the stack of layers with luminescent material, in other words, the stack of layers with luminescent material is arranged at a specific distance away from the light source.
The stack of luminescent of layers of the embodiments of the cited patent application have the layer with the organic luminescent material arranged at a side of the stack of layers at which the viewer looks towards the phosphor-enhanced light source. The layer with the inorganic luminescent material is arranged at a side of the stack of layers that is closest to the light emitting surface of the light source. The layer with organic luminescent material is often relatively transparent because the organic luminescent molecules are intrinsically transparent. A viewer does not see a color in the layer with organic luminescent material. However, inorganic luminescent particles in the layer with inorganic luminescent material act as a scattering material and reflect light in all directions. Thus, if light from the ambient impinges on the luminescent material, some light in the absorption spectrum of the inorganic luminescent material is absorbed, and the non-absorbed light is reflected and/or scattered, which, consequently, has a color that is a complement color of the color of the absorption spectrum of the inorganic luminescent material. This complement color is seen by a viewer who looks towards the stack of luminescent layers. This is termed “a color appearance”, which means that the layer appears to have a specific color. In many applications it is undesired to have the color appearance because it seems for the viewer that a phosphor-enhanced light source has the specific color of the light that is generated by the inorganic luminescent material. For example, if a specific inorganic luminescent material has an absorption spectrum in the blue color range, the color appearance of the specific organic luminescent material will be orange/yellow.
Further, the light sources of the cited patent application have to emit light with a correlated color temperature typically in a range of 2700 to 6000 Kelvin, and typically have a blue emitting Light Emitting Diode (LED). To obtain this correlated color temperature, a relatively large amount of blue light has to be converted to light of a longer wavelength. The conversion of blue light to light of a longer wavelength is performed by the inorganic luminescent material, and thus a relatively large amount of inorganic luminescent material needs to be used, which results in a stronger color appearance.
It is to be noted that it is known that organic luminescent material may be used instead of inorganic luminescent material. However, in order to obtain a high enough color rending index it may be necessary to use a combination of an organic luminescent material and an inorganic luminescent material.